Light element geochemistry and spallogenesis in lunar rocks     

David J.Des Marais 《Geochimica et cosmochimica acta》1983,47(10):1769-1781

The abundances and isotopic compositions of carbon, nitrogen and sulfur were measured in eleven lunar rocks. Samples were combusted sequentially at three temperatures to resolve terrestrial contamination from indigenous volatiles.Sulfur abundances in Apollo 16 highland rocks range from 73 to 1165 μg/g-whereas sulfur contents in Apollo 15 and 17 basalts range from 719 to 1455 μg/g and correlate with TiO₂ content. Lunar rocks as a group have a remarkably uniform sulfur isotopic composition, which may reflect the low oxygen fugacity of the basaltic magmas. Much of the range of reported δ³⁴S_cd values (?2 to + 2.5 permil) is caused by systematic analytical discrepancies between laboratories.Lunar rocks very likely contain less than 0.1 μg/g of nitrogen. The measured spallogenic production rate, 4.1 × 10^?6 μg ¹⁵N/g sample/m.y., agrees remarkably closely with previous estimates. An estimate which includes all available data is 3.7 × 10^?6 μg¹⁵N/g sample/m.y.Lunar basalts may contain no indigenous lunar carbon in excess of procedural blank levels (~0.7 μg/g). Highlands rocks consistently release about 1 to 5 μg/g of carbon in excess of blank levels, but this carbon might either derive from ancient meteoritic debris or be a mineralogie product of terrestrial weathering. The average measured spallogenic ¹³C production rate is 4.1 × 10^?6 μg¹³C/g sample/m.y. The ¹³C spallation exposure ages of rocks 15058 and 15499 are 184 and 135 m.y., respectively.  相似文献   

From species abundance to opal input: Simple geometrical models of radiolarian skeletons from the Atlantic sector of the Southern Ocean     

H. Jacot Des Combes  A. Abelmann 《Deep Sea Research Part I: Oceanographic Research Papers》2009,56(5):757-771

Radiolarian-based paleoceanographic reconstructions generally use the abundance of selected radiolarian species. However, the recent focus on the opal flux and the development of isotope measurements in biogenic opal and the organic matter embedded in it demands a better knowledge of the origin of the opal. We present here an estimation of the opal content of the skeleton of 63 radiolarian species from two sites in the Southern Ocean. The skeletons are modelled as associations of simple geometrical shapes, and the volume thus obtained is combined with opal density to obtain the amount of opal. These data are, thus, used to determine the most important opal carriers in the radiolarian assemblage in both cores.  相似文献   

A simple fiber-optic microprobe for high resolution light measurements: application in marine sediment     

Jorgensen BB  Des Marais DJ 《Limnology and oceanography》1986,31(6):1376-1383

A fiber-optic microphobe is described which is inexpensive and simple to build and use. It consists of an 80-micrometers optical fiber which at the end is tapered down to a rounded sensing tip of 20-30-micrometers diameter. The detector is a hybrid photodiode/amplifier. The probe has a sensitivity of 0.01 microEinst m-2 s-1 and a spectral range of 300-1,100 nm. Spectral light gradients were measured in fine-grained San Francisco Bay sediment that had an undisturbed diatom coating on the surface. The photic zone of the mud was only 0.4 mm deep. Measured in situ spectra showed extinction maxima at 430-520, 620-630, 670, and 825-850 nm due to absorption by chlorophyll a, carotenoids, phycocyanin, and bacterio-chlorophyll a. Maximum light penetration in the visible range was found in both the violet and the red < or = 400 and > or = 700 nm.  相似文献   

Use of exposure time and life expectancy in models for toxicity to aquatic organisms     

Connell D  Yu J 《Marine pollution bulletin》2008,57(6-12):245-249

The exposure time is a variable which is usually not incorporated into models for toxicity. However, with persistent organic pollutants (POPs) exhibiting a nonspecific mode of action and lipophilic properties this variable can be modeled by the usage of the internal concentrations as a measure of the toxicity with fish. The bioconcentration process with fish is a relatively well understood and predictable process which allows the calculation of the internal lethal concentration. When the exposure time is relatively short the critical internal lethal concentrations are relatively constant for the group of POPs whereaas the LC50 measured in the ambient water is quite variable. When the exposure time is relatively long, results on the measurement of the critical internal concentration with fish over different exposure times has demonstrated that the internal lethal concentration falls with increasing exposure times in a consistent and predictable manner. This reduction in life expectancy can be described in a model which can be used to estimate the critical internal concentration for any exposure time. It also provides information useful in assessing the risk to fish and potentially other species due to the occurrence of residues of POPs in natural aquatic systems. It is suggested that these relationships can be extended to other groups of organisms and chemicals.  相似文献   

The diffusive boundary layer of sediments: oxygen microgradients over a microbial mat     

Jorgensen BB  Des Marais DJ 《Limnology and oceanography》1990,35(6):1343-1355

Oxygen microelectrodes were used to analyze the distribution of the diffusive boundary layer (DBL) at the sediment-water interface in relation to surface topography and flow velocity. The sediment, collected from saline ponds, was covered by a microbial mat that had high oxygen consumption rate and well-defined surface structure. Diffusion through the DBL constituted an important rate limitation to the oxygen uptake of the sediment. The mean effective DBL thickness decreased from 0.59 to 0.16 mm as the flow velocity of the overlying water was increased from 0.3 to 7.7 cm s-1 (measured 1 cm above the mat). The oxygen uptake rate concurrently increased from 3.9 to 9.4 nmol cm-2 min-1. The effects of surface roughness and topography on the thickness and distribution of the DBL were studied by three-dimensional mapping of the sediment-water interface and the upper DBL boundary at 0.1-mm spatial resolution. The DBL boundary followed mat structures that had characteristic dimensions > 1/2 DBL thickness but the DBL had a dampened relief relative to the mat. The effective surface area of the sediment-water interface and of the upper DBL boundary were 31 and 14% larger, respectively, than a flat plane. Surface topography thereby increased the oxygen flux across the sediment-water interface by 49% relative to a one-dimensional diffusion flux calculated from the vertical oxygen microgradients.  相似文献   

The framing of climate change and development: A comparative analysis of the Human Development Report 2007/8 and the World Development Report 2010     

Des Gasper  Ana Victoria Portocarrero  Asuncion Lera St.Clair 《Global Environmental Change》2013,23(1):28-39

The Human Development Report 2007/8 (HDR) and the World Development Report 2010 (WDR) are both devoted to the connections between climate change and development. The reports provide very different perspectives on where the key challenges reside. Their policy proposals are also different, but much less so. Using a combination of frame and content analysis complemented with attention to how these institutions and their knowledge production processes operate, the paper develops a structured comparison of the problem-framing and solution-framing in the Overviews of the two reports. It compares the reports’ conceptions of development; their normative content and the roles given to human rights; the pathways and solutions the agencies defend, and how protective they are of the poor; the status given to proposed market solutions; what types of information and expert perspective are prioritized and why, and which issues are neglected. Following its stress on human rights and ethical principles, the HDR does not unsettle the market driven solutions to climate change that dominate the global policy arena. Different policy instruments and a fuller institutionalization of its human rights concerns is required, in organizations, processes, methodologies and knowledge networks that could move the ideas forward and continually press for their use. This may require a different model of report preparation.  相似文献   

Characteristics and OSL minimum ages of relict fluvial deposits near Sossus Vlei,Tsauchab River,Namibia, and a regional climate record for the last 30 ka     

George A. Brook  Pradeep Srivastava  Eugene Marais 《第四纪科学杂志》2006,21(4):347-362

The present end‐point of the Tsauchab River is at Sossus Vlei, 30 km into the Namib Sand Sea. Interdune deposits in three depressions west and southwest of the vlei include channel and interdune lithofacies associations but no deposits typical of river end‐points or of groundwater seepage into interdune areas. The two lithofacies associations show that the Tsauchab River extended further into the sand sea in the past. It had a well‐developed channel and a higher flow than today that caused flooding of adjacent interdune areas. OSL 4‐mm aliquot minimum ages indicate that the Tsauchab River reached 2–3 km beyond its present end‐point at ca. 25 ka and ca. 9–7 ka, and that the river was more active from 0.9–0.3 ka. The eastward migration of the river end‐point since ca. 7 ka suggests a reduction in flood magnitude accompanied by the gradual invasion of the Sossus Vlei area by dunes. The regional data indicate an additional wet interval at ca. 15 ka that is so far not recorded in the Sossus area. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Editorial     

Malcolm G. Anderson  Des E. Walling 《水文研究》1993,7(1):i-i

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Environmental distribution of chlorohydrocarbon contaminants associated with Sydney sewage discharges     

Munro R. Mortimer  Des W. Connell 《Marine pollution bulletin》1996,33(7-12):160-167

A steady-state nonequilibrium fugacity model incorporating the oceanographic dimensions of the environment receiving a sewage discharge from Sydney, Australia was used to predict the environmental fate of a range of chlorohydrocarbon contaminants. The model showed advection in water to be the dominant removal process, accounting for all except 1–2% of the contaminants discharged. With the exception of the more water soluble dieldrin, the major portion of contaminant present in the area was held in the sediments. Bioconcentration and sediment sorption models were used with levels detected in the biota and sediments to calculate the ambient water concentrations of the chlorohydrocarbon contaminants. The ambient water concentrations calculated using these models were in general agreement within a factor of about 2 in all but one case, and that was in agreement within an order of magnitude. The fugacity model was also used to estimate the rates of discharge of each of the contaminants. These were chlordane 83–640, DDT 150, dieldrin 1400–1500, hexachlorobenzene 360–400 and PCBs 55–96 g day⁻¹. Comparisons of the calculated ambient concentrations of each compound in the water and in fish in waters adjacent to the discharge point before and after its relocation from a cliff-face to a deepwater site indicated that relocation should result in an order of magnitude reduction.  相似文献   

Modelling floodplain flows using a two-dimensional finite element model     

Paul D. Bates  Malcolm G. Anderson  Laura Baird  Des E. Walling  David Simm 《地球表面变化过程与地形》1992,17(6):575-588

A prototype two-dimensional finite element flow model for depth-averaged free surface flows was developed for floodplain environments. Limited refinement of the model's physical representation was undertaken and the enhanced scheme applied to an 11 km river channel/floodplain reach in the U.K. Preliminary model results indicate that this modelling approach can be used to identify dynamic variations in the flow field parameters over length scales of the order of 10-100 m. Potentially, such data have the ability to permit detailed analysis of short-term floodplain sedimentary dynamics.  相似文献